GPR measurements to assess the Emeelt active fault's characteristics in a highly smooth topographic context, Mongolia

Abstract

To estimate the seismic hazard, the geometry (dip, length and orientation) and the dynamics (type of displacements and amplitude) of the faults in the area of interest need to be understood. In this paper, in addition to geomorphologic observations, we present the results of two ground penetrating radar (GPR) campaigns conducted in 2010 and 2011 along the Emeelt fault in the vicinity of Ulaanbaatar, capital of Mongolia, located in an intracontinental region with low deformation rate that induces long recurrence time between large earthquakes. As the geomorphology induced by the fault activity has been highly smoothed by erosion processes since the last event, the fault location and geometry is difficult to determine precisely. However, by using GPR first, a non-destructive and fast investigation, the fault and the sedimentary deposits near the surface can be characterized and the results can be used for the choice of trench location. GPR was performed with a 50 MHz antenna over 2-D lines and with a 500 MHz antenna for pseudo-3-D surveys. The 500 MHz GPR profiles show a good consistency with the trench observations, dug next to the pseudo-3-D surveys. The 3-D 500 MHz GPR imaging of a palaeochannel crossed by the fault allowed us to estimate its lateral displacement to be about 2 m. This is consistent with a right lateral strike-slip displacement induced by an earthquake around magnitude 7 or several around magnitude 6. The 2-D 50 MHz profiles, recorded perpendicular to the fault, show a strong reflection dipping to the NE, which corresponds to the fault plane. Those profiles provided complementary information on the fault such as its location at shallow depth, its dip angle (from 23 • to 35 •) and define its lateral extension.

Geophysical characterization of buried active faults: the Concud Fault (Iberian Chain, NE Spain)

“…The results illustrate the usefulness of geophysics to detect and map non-exposed faults in areas of moderate-to-low tectonic activity where faults are often covered by recent pediments that obscure geological evidence of the most recent earthquakes. The results also highlight the importance of Štěpančíková et al 2011;Fischer et al 2012;Dujardin et al 2014). …”

Tectonic Geomorphology and Paleoseismology of the Sharkhai fault: a new source of seismic hazard for Ulaanbaatar (Mongolia)

“…Between 2005 and 2019, more than 10 swarm episodes of moderate earthquakes (M ≤ 4.5) have been recorded and accurately relocated ∼ 10 km west of the capital (Adiya, 2016). Tectonic geomorphology investigations focused on the swarm area revealed evidence of Quaternary activity along the Emeelt fault (Ferry et al, 2010;Schlupp et al, 2010a;Ferry et al, 2012;Schlupp et al, 2012;Dujardin et al, 2014). This structure is located near the eastern end of the Hustai fault, strikes N140 (Fig.…”

Fault Structure Reconstruction From Magnetic Anomalies Using an Improved Backtracking Search Optimization Algorithm

“…Therefore, the characterization of active faults is one of the most important research topics in geophysics. For this purpose, various geophysical prospecting methods such as electrical resistivity (Arjwech et al., 2021), gravity (Hiramatsu et al., 2019), magnetic (Khalil, 2016), electromagnetic (Martí et al., 2020), self‐potential (Saribudak & Hauwert, 2017), and ground‐penetrating radar (Dujardin et al., 2014) are commonly used (Spahić et al., 2010). The magnetic method measures variations in the Earth's magnetic field caused by changes in the magnetic properties of the rocks.…”